Check the picture below.
we know that the arcST is 30°, meaning the inscribed angle intercepting it will be half that or 15°.
in the triangle RWT, two sides of it are RT and RW, both of which are radius segments and thus equal, meaning that triangle RWT is an isosceles, and in an isosceles the twin sides also make twin angles, meaning that ∡RTW is a twin of the inscribed angle ∡RWT.
